1. Field of the Invention
The present invention relates to an optical transmission apparatus and an optical transmission control method, and more particularly to an optical transmission apparatus and an optical transmission control method, for amplifying and transmitting a wavelength-division-multiplexed optical signal.
2. Description of the Related Art
In a wavelength-division-multiplexing optical transmission system, an optical signal with each wavelength needs to be kept at constant power in order to maintain high-quality transmission characteristics. There are mainly two factors in a power variation of a wavelength-division -multiplexed optical signal, which factors are (1) a change in the number of wavelengths and (2) a change in the transmission loss. When the number of wavelengths is changed, the sum power of all optical signals is changed, but the power of the optical signal with each wavelength is not changed. On the other hand, when the transmission loss is changed, both the sum power of all optical signals and the power of the optical signal with each wavelength are changed. Because of these differences, upon the occurrence of the power variation, it is necessary to change the control of the transmitted optical signal. Specifically, when the number of wavelengths is changed, the power of the optical signal with each wavelength is not changed, and hence, the power of the optical signal with each wavelength need not be changed. On the other hand, when the transmission loss is changed, the power of the optical signal with each wavelength is changed, and hence, the power of the optical signal with each wavelength needs to be changed.
In recent years, the number of cases where the optical repeating office is provided with a function for adding/dropping an optical signal with an arbitrary wavelength has been increasing in the wavelength-division-multiplexing optical transmission system. This makes it possible to easily increase or decrease the number of the transmitted wavelengths. Here, the assumption is made that a power variation is caused in a wavelength-division-multiplexed optical signal inputted in an optical repeating office. In this case, it is necessary to detect whether the power variation is caused by the change in the number of wavelengths or by the variation in the transmission loss, in order to maintain high-Quality transmission characteristics. A technique related to this case is disclosed in Japanese Patent Laid-Open No. 9-219696.
The optical amplifying apparatus herein disclosed amplifies each signal within the wavelength-division -multiplexed optical signal all together. In the amplifying operation, the optical amplifying apparatus controls the average value of optical signal levels of each wavelength (channel) to be constant against the change in the number of inputted wavelengths (channels) and the variation in the level of inputted optical signals. Specifically, the optical amplifying apparatus is configured so that two optical fiber amplifiers which perform automatic gain control (AGC) are cascade-connected and a variable optical attenuator is inserted between the optical fiber amplifiers. Thus, the optical amplifying apparatus detects the optical signal levels and the wavelength-division-multiplexed number (the number of channels). Further, the optical amplifying apparatus obtains the average value of the optical signal levels of each wavelength (each channel) and controls an attenuation amount of the variable optical attenuator so as to make the average value remain constant. The detection of the wavelength -division-multiplexed number is performed by a wavelength -division-multiplexed number detecting circuit. The calculation of the average value of optical signal levels is performed by an average level detecting circuit.
In this related technique, the wavelength-division -multiplexed number detecting circuit needs to be provided in order to monitor the wavelength-division-multiplexed number. The wavelength-division-multiplexed number detecting circuit demultiplexes a wavelength-division -multiplexed optical signal to each wavelength and then detects the presence or absence of each wavelength. Further, in this related technique, the average level detecting circuit also needs to be provided in order to obtain the average value of optical signal levels of each wavelength. This related technique needs such expensive circuits, and hence has a problem that the cost is high.
Thus, there is a method in which information on the number of wavelengths is received by a supervisory signal for transmission line (SV light). A technique related to this method is disclosed in Japanese Patent Laid-Open No. 10-51057 (patent family: U.S. Pat. No. 6,157,481, U.S. Pat. No. 6,198,572, and U.S. Pat. No. 6,144,485). In this related technique, a control signal which notifies a change in the number of channels (the number of wavelengths), is multiplexed onto the wavelength-division -multiplexed optical signal and transmitted. Then, the optical amplifying apparatus extracts the transmitted control signal and thereby receives the change in the number of channels of the optical signals. However, it takes much time until the optical amplifying apparatus to receive the control signal from the time when the control signal is transmitted from an apparatus in the preceding stage. For example, it takes about several seconds, and when the time period is long, it takes about several tens seconds. Therefore, a delay time is generated between the time when the number of wavelengths of optical signals is changed and the time when the information on the changed number of wavelengths is received by the optical amplifying apparatus. During this period of time, proper control is not performed, and hence, the output power per channel (wavelength) may be deviated from a target value so as to cause deterioration in the transmission quality. Therefore, this related technique, in which timely control cannot be performed, has a problem in reliability.